1. Field of Invention
The present invention relates to pressure wave transducers and more particularly to a pressure wave sensor of significant length employing optical fibers and utilizing the principle of microbending.
2. The Prior Art
Pressure sensors or transducers of significant length are described in a number of U.S. Patents U.S. Pat. No. 2,965,877 describes a hydrophone with capacitive effect in which the capacitance between the two armatures of a capacitor varies in relation to the pressure applied to the sensor.
U.S. Pat. No. 3,317,891 also describes a hydrophone of continuous structure wherein the variation of resistance of conductors forming part of a sensor are converted to electrical signals.
Yet another elongated pressure wave transducer is described in U.S. Pat. No. 3,798,474 as being capable of measuring the pressure variations resulting from seismic vibrations either in the sea or on the earth and being of the piezoelectric type. This transducer consists of a long strip of flexible material having piezoelectric properties and the faces of which are each associated with an electric conductor.
Of course, the best known elongated pressure wave transducer in use today by commercial marine operators in seismic surveying consists of detector arrays or stations comprising a plurality of rigid hydrophone structures mounted within a cable with each array extending for 25 to 50 meters and numbering many individual hydrophones.
One characteristic common to all the foregoing transducers and arrays is the fact that they rely upon the use of or the generation of electrical energy, features which in the art characterize them as active transducer elements.
Recently a suggestion has been made, as represented by U.S. Pat. No. 4,115,753, to employ a fiber optic acoustic array using optical hydrophones which sense sound waves. The light signals so generated are transmitted along a fiber-optic bundle. As described in this patent each of the optical hydrophones is separately coupled to a separate one of the multitude of optical fibers. The sensed signal from each of the hydrophones is transmitted by way of its own optical fiber to a photodetector, there being a plurality of photo detectors each coupled to a corresponding hydrophone. This then amounts to an optical system equivalent to the more conventional data gathering systems in that each hydrophone has its own connection by way of an optical fiber to recording equipment on board a vessel.